polymers Article The Effect of Conformation Order on Gas Separation Properties of Polyetherimide Ultem Films Julia Kostina 1,*, Sergey Legkov 1, Alexander Kolbeshin 1, Roman Nikiforov 1, Denis Bezgin 1, Alexander Yu. Nikolaev 2 and Alexander Yu. Alentiev 1,* 1 A.V. Topchiev Institute of Petrochemical Synthesis Russian Academy of Sciences, 119991 Moscow, Russia; [email protected] (S.L.); [email protected] (A.K.); [email protected] (R.N.); [email protected] (D.B.) 2 A.N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences, 119334 Moscow, Russia; [email protected] * Correspondence: [email protected] (J.K.); [email protected] (A.Y.A.) Received: 16 June 2020; Accepted: 7 July 2020; Published: 16 July 2020 Abstract: Changes of the spectral characteristics of absorption bands depending on the films’ treatment method were registered for polyetherimide Ultem films. The possibility of selection of structural criteria (the ratio of the functional groups absorption bands intensities) showing all conformational changes in elementary unit with metrological processing of the results is shown. It is demonstrated that film formation from chloroform solution leads to elementary unit fragments, Ph–O–Ph0, which have an effect on macromolecule conformation and result in increasing of space between fragments of macromolecules (local polymer matrix packing loosening). Desorption of residual chloroform from films by ethanol or supercritical CO2 leads to a change of conformers set in Im–Ph–Im0 units. Quantum chemical modeling showed the possibility of convergence of these fragments in neighboring macromolecules, and consequently of interchain π–π interaction (local densification of chain packing of the polymer matrix). After annealing at a temperature higher than glass transition temperature, the polyetherimide film exhibits the most disordered (amorphous) state at all of the fragments. It is demonstrated that the results, obtained by the combination of theoretical and experimental vibrational spectroscopy methods, are in good agreement with data of chain packing ordering found by analysis of gas separation parameters. Keywords: gas separation; polyetherimide; FTIR spectroscopy; conformational order; quantum chemical calculations; non-covalent interactions; residual solvent 1. Introduction One of the key problems of the application of glassy amorphous polymers is a dependence of sample properties and its history. This is due to the thermodynamic nonequilibrium of the glassy state. Therefore, glassy amorphous polymers are used in different stationary states which depend on preparation technique and operation mode. Especially, significant effects occur for thin films and coatings. For example, glassy amorphous polymers are applied as materials for thin continuous layers (several tens up to several hundred nanometers) of nonporous gas separation membranes. Therefore, specifically for membrane material science, the stability of gas separation properties in time and the effect of preparation technique and operation mode of the polymer membrane (film) on its gas separation characteristics are very important issues [1]. As a rule, polymer membranes are made out of polymer solutions, thus gas separation properties are sensitive to solvent type and polymer solution concentration [2–5], presence of residual solvent, and method of its removal [6–9]. Thermodynamic nonequilibrium state of glassy amorphous polymers leads to a change in nonequilibrium free volume and consequently gas separation properties of polymer membranes during exploitation due to physical Polymers 2020, 12, 1578; doi:10.3390/polym12071578 www.mdpi.com/journal/polymers Polymers 2020, 12, 1578 2 of 15 aging [1,9,10] or plasticization with a separated component [1,11]. On the other hand, this is why gas separation characteristics might be considered as a nonequilibrium level assessment criterion for polymer film, prepared by one method or another [12]. As there are no direct methods of measurement for chain packing ordering for X-ray amorphous polymers, a combination of analysis of gas diffusion coefficients [12] with the results of other methods can be promising for the determination of a trend for a change of chain packing ordering and the effect of this trend on physicochemical and transport properties. One of these methods is vibrational spectroscopy, which provides evaluation of conformational structure change on atomic level and molecular level. An additional factor supporting the selection of vibrational spectroscopy as independent method of assessment of tendency of treatment technique effect on conformational structure of X-ray amorphous polymers is a well-developed theory of vibrational spectroscopy. It allows one to use both quantum chemistry methods to interpret experimental data and alternatively to substantiate reliability of theoretical calculations by results generated by test samples IR spectra analysis. Using a combination of experimental and theoretical methods of vibrational spectroscopy, and based on the results of an analysis of changes of test samples spectra, it is now also possible to resolve the inverse problem of vibrational spectroscopy for polymers, i.e., to register changes of elementary unit structure based on the model justified by experimental spectral data. This approach can be successfully applied to calculate interactions, including noncovalent interactions. Comparison of theoretical vibrational spectra with experimental ones can allow one to make adequate conclusions and to find relationship between structural (particularly, conformational) changes in macromolecule and polymeric object properties altogether. Previously, in [8,13–16], it has been shown that interaction between functional groups of polymer with a solvent with formation of hydrogen bonds leads to reorientation of these groups over the polymer backbone, changing its conformational parameters [13–16]. If hydrogen bond formation leads to an increase in the quantity of elementary unit conformers of the same geometry, it is appropriate to suppose ordering of conformational composition of elementary units of macromolecule, which results in chain packing ordering. In turn, polymer chain packing ordering may lead to modification of free volume elements distribution in the matrix [15]. In these cases, the modes ratio in bimodal free volume elements (FVE) size distribution changes with an increase of specific mode concentration. Such modification of FVE size distribution is accompanied by a sharp increase in gas separation selectivity with maintaining [13,15] of gas permeability (sometimes these data exceed upper bound [15]) of permeability-selectivity diagram [17], which makes polymer material perspective for membrane gas separation. In [15], the relationship of gas separation parameters with conformational chain ordering when the polymer is treated with non-solvents (alcohols) also forming complexes with hydrogen bonds was also considered. Gas separation selectivity growth with gas permeability increase has also been observed for polyetherimides (PEI) films swelling in supercritical CO2 (sc-CO2) with further slow depressurization (0.008–0.025 MPa/min) [18–20]. It is well known that polymer treatment with sc-CO2 facilitates desorption of a residual solvent [21] independently of its capability to form hydrogen bonds with polymer functional groups. However, according to the authors of [12], such film treatment can lead to increase of chain packing ordering in polyetherimides. Nevertheless, there has been no confirmation of changes of conformational chain ordering after amorphous polymer film treatment with sc-CO2 obtained by other methods. The aim of this work is to verify the hypothesis on the effect of different film formation and treatment conditions on conformational order of macromolecules. Quantitative criteria of amorphous polymer chain packing ordering are proposed for PEI Ultem (Scheme1) based on analysis of structural characteristics from IR spectra of polymeric films at di fferent formation and treatment conditions and diffusion coefficients of non-condensable gases (H2, He, N2, O2, CO2, and CH4). Formation of hydrogen bonds has been noted earlier for this polymer [13,15], and quantitation of chain packing ordering change has been performed using gas separation characteristics analysis [12] at different film formation and treatment conditions. Polymers 2020, 12, 1578 3 of 15 Polymers 2020, 12, x FOR PEER REVIEW 3 of 15 O CH3 O O C O N N CH3 n O O SchemeScheme 1.1. ElementaryElementary unitunit ofof UltemUltem structurestructure ((TTgg == 215 °C).◦C). 2. Materials and Methods 2. Materials and Methods 2.1. Film Formation 2.1. Film Formation All Ultem PEI films (ULTEM® 1000 Resin, SABIC Innovative Plastics, Riyadh, Saudi Arabia) were All Ultem PEI films (ULTEM® 1000 Resin, SABIC Innovative Plastics, Riyadh, Saudi Arabia) formed from a 5% solution in chloroform (ChP). were formed from a 5% solution in chloroform (ChP). For gas separation properties investigation, films of 30–35 µm thickness were prepared by casting For gas separation properties investigation, films of 30–35 µm thickness were prepared by over a cellophane support from 5% solution in chloroform with subsequent drying for 2–3 days at casting over a cellophane support from 5% solution in chloroform with subsequent drying for 2–3 ambient temperature and then keeping them in vacuum at 1–2 mmHg until constant weight is achieved. days at ambient temperature and then keeping them in vacuum at 1–2 mmHg until constant weight Films for investigation